Oil well pipes are commonly connected to each other by pin-box type threaded joints comprising a pin having an external thread and a box having an internal thread. Pin-box type threaded joints can be classified as coupling types and direct connection types.
In coupling-type threaded joints, normally, both ends of an oil well pipe have a pin comprising an external thread formed thereon. The pin of an oil well pipe can be connected to the pin of another oil well pipe by a coupling, which is a separate member having an internally-threaded box formed thereon.
In direct connection type threaded joints, a pin having an external thread is formed on one end of an oil well pipe, and a box having an internal thread is formed on the other end. Two oil well pipes are connected to each other by screwing the pin of one pipe into the box of the other pipe and tightening the joint.
In order to improve the sealing properties of a threaded joint, in recent years, a special threaded joint having an unthreaded metal contact portion which adjoins a threaded portion and which can form a metal-to-metal seal has come to be used.
In general, when a threaded joint is tightened, in order to reduce friction, a lubricating oil is applied to a portion of the joint. In particular, with threaded joints having an unthreaded metal contract portion to which it is necessary to apply an extremely high surface pressure in order to guarantee sealing properties, it is customary to apply to the joint a lubricating grease which is viscous at room temperature in order to prevent galling, which is unrepairable seizing.
In order to increase the retention of a lubricating oil or a lubricating grease, a threaded joint is frequently subjected to phosphate treatment with manganese phosphate, zinc phosphate, or similar material. The crystalline phosphate coating which is formed is porous and has numerous pores, so it can retain a lubricating oil or grease in its pores. When pressure is applied to a threaded joint during tightening, the phosphate coating is compressed to cause the lubricating oil or grease retained in its pores to be released out of the coating. Accordingly, by forming a phosphate coating on the surface of a threaded joint, the rust-preventing properties and lubricity of a lubricating oil or grease are greatly improved.
This effect can be obtained by forming a phosphate coating on the surface of either the pin or the box of a threaded joint and applying a lubricating oil or grease to the coating. It is desirable that the phosphate coating be formed with a uniform thickness (or coating weight). If there are variations in the thickness of a phosphate coating, the amount of lubricating oil or grease which is retained by the phosphate coating varies. As a result, in locations where the phosphate coating is thin, galling may occur due to insufficient lubricity (galling occurring particularly readily in unthreaded metal contact portions).
Methods commonly used for performing surface treatment such as phosphate treatment of threaded joints formed on the ends of steel pipes such as oil well pipes include the immersion method and the dropping method. In the immersion method, as schematically illustrated in FIG. 7, surface treatment is carried out by disposing a steel pipe 3 in a tilted state with a threaded portion formed on one of its ends immersed in a treatment liquid 2 contained in a tank 1. In the dropping method, as schematically illustrated in FIGS. 8(a) and 8(b), a treatment liquid 2 in a tank (not shown) is passed through a supply pipe 5 and is sprayed or dripped from nozzles 4 onto a threaded portion 3a on the end of a steel pipe 3.
The immersion method requires a large installation space, and it is necessary to lift and lower a steel pipe being treated, so its working efficiency is poor. In addition, when performing surface treatment only of the exterior surface of the end of a steel pipe, it is necessary to close the open end of the pipe to prevent the treatment liquid from entering its interior, or it is necessary to perform treatment to prevent the treatment liquid from adhering to the interior of the pipe, so the method becomes complicated to perform.
In the dropping method, as shown in FIGS. 8(a) and 8(b), a plurality of steel pipes can simultaneously undergo continuous surface treatment by arranging the pipes in parallel and spraying a treatment liquid from a plurality of nozzles onto the pipes as the pipes are being conveyed, so surface treatment can be efficiently performed in a smaller space.
Phosphate treatment is preceded by pretreatment including degreasing and subsequent water rinsing, and followed by post-treatment including rinsing with cold and/or warm water to remove excess treatment liquid. In order to efficiently carry out phosphate treatment including this pretreatment and post-treatment by the dropping method, Japanese Patent No. 2,988,310 discloses an apparatus in which the ends of steel pipes are successively passed beneath nozzles which supply liquids for each step as the steel pipes are conveyed at a constant speed in a direction perpendicular to the pipe axes while being rotated. Different treatment areas are partitioned from each other by curtains in order to prevent the liquids which are dropped in each step from mixing with each other.
In the dropping method, when a treatment liquid is sprayed from nozzles as shown in FIGS. 8(a) and 8(b), the treatment liquid spreads out in a conical shape, so the closer the nozzles are to the steel pipes being treated, the easier it is for the amount of the treatment liquid which is sprayed onto the pipe surface to become nonuniform. As a result, in the case of phosphate treatment, variations develop in the thickness of the phosphate coating which is formed, and “lack of hiding”, which is a condition in which the bare metal of the steel pipe is visible, can easily occur in portions where the coating thickness is low. In addition, when the supply pipe 5 and the nozzles 4 are arranged in one row as shown in the figures, the area of treatment measured in the axial direction of the steel pipes varies in accordance with the separation (distance) between the nozzles and the steel pipes. Therefore, if the nozzles are too close to the steel pipes, in order to form a phosphate coating over a desired region in the axial direction of the pipes, it becomes necessary to take steps such as providing supply pipes and nozzles in two rows, and the treatment apparatus becomes complicated.
Phosphate treatment is usually carried out at a temperature higher than room temperature in order to promote the reaction on the surface of the steel pipe. In the case of the dropping method, a phosphate treatment liquid (a phosphating solution) which is heated in a tank to a temperature of 70±5° C. is passed through the supply pipe and sprayed from the nozzles, whereby phosphate treatment is performed at a temperature above room temperature. If the temperature of the phosphating solution on the surface of the steel pipe changes, the coating weight and the crystallinity of the resulting phosphate coating also change.
The treatment liquid which is sprayed from the nozzles decreases in temperature due to contact with the atmosphere which is at a cooler temperature. When sprayed from the nozzles, the treatment liquid is formed into fine droplets, and its surface area is increased, so it undergoes a large decrease in temperature at this time. Therefore, when the diameter of the steel pipe being treated varies, the distance between the nozzles and the steel pipe also varies, and the extent of decrease in the temperature of the treatment liquid varies. Thus, in the case of phosphating, the temperature of the phosphating solution when it reaches the surface of the steel pipe varies, and as a result, a phosphate coating having a desired coating weight and crystallinity may not be formed. Similarly, when the distance between the nozzles and the steel pipe is increased in order to increase the length in the axial direction of the pipe over which treatment is performed, the extent of decrease in temperature also increases, and a phosphate coating having a desired coating weight and crystallinity may not be formed.
If the temperature of the phosphating solution in the tank is increased in order to compensate for the above-described decrease in temperature by spraying, the phosphating solution becomes overheated, leading to the formation of precipitates (sludge). As a result, active ingredients of the phosphating solution are consumed as sludge, resulting in increased costs of the solution.
Thus, particularly in the dropping method in which a phosphating solution is sprayed from nozzles, it was difficult to suitably control the temperature of the solution when it contacted the surface of a steel pipe. As a result, the coating weight or the crystallinity of the phosphate coating which was formed were inadequate, or the coating weight was not uniform, and when a lubricating oil or grease was applied to it, the above-described problems developed.
As a solution to cope with these problems, Japanese Patent No. 2,660,689 discloses a method in which a phosphating solution is allowed to naturally fall by overflowing as a laminar flow from a current plate provided in a tank and contact the surface of rotating steel pipes.
However, in that method, it is not possible to simultaneously treat a plurality of steel pipes using a single tank, so it has a lower treatment efficiency than treatment by the method shown in FIGS. 8(a) and 8(b) in which a plurality of steel pipes can be simultaneously treated by spraying from nozzles. Furthermore, the position of the tank is fixed, so depending upon the outer diameter of the pipe being treated, the distance between the tank and the pipe increases, the temperature of the phosphating solution decreases, and the coating weight of the phosphate coating decreases. Furthermore, in that method, it is mandatory to rotate the steel pipes while dropping a phosphating solution thereon.